Method and apparatus for improving formation uniformity of paper

ABSTRACT

TO IMPROVE THE FORMATION PROPERTIES OF A NONWOVEN WEB OF CELLULOSIC FIBER, THE WEB IS DRIED TO A MOISTURE CONTENT OF BETWEEN 18-40% MOISUTRE AND ADVANCED THROUGH A PRESSURE NIP BETWEEN A PAIR OF OPPOSED ROLLS. THE ROLLS ARE EACH VERY SLIGHTLY RESILIENT BUT HAVE A HARD SURFACE OF BETWEEN 3 AND 10 P &amp; J PLASTOMETER 1/8 &#34; BALL). THE PRESSURE NIP COMPACTION ALSO CONTRIBUTES TO DENSIFICATION, IMPROVED SURFACE SMOOTHNESS AND IMPROVED UNIFORMITY OF FIBER DISTRIBUTION.

March 7, 1972 s, 5;, K ETAL 3,641,618

METHOD AND APPARATUS FOR IMPROVING FORMATION, UNIFORMITY OF PAPER Filed Aug. 18, 1969 I 2 Sheets-Sheet'2 'IVC) 290' oosz WITH INVENTIVE TREATMENT WITHOUT INVENTIVE TREATMENT 7262524 ???I20|9|8 I7 |6l5|4l3 2H I09 6 7 6 5 4 3 2| FLOC SEPARATION!) a (INCHES) F REQUE NCY (CPS) a I I SANGHO E. BACK RUEBEN A. MART! INVENTORS FIG.

United States Patent 3 647 618 METHOD AND APPARATUS FOR IMPROVING FORMATION UNIFORMITY OF PAPER Sangho E. Back and Rueben A. Marti, Vancouver,

Wash., assignors to Crown Zellerbach Corporation, San

Francisco, Calif.

Filed Aug. 18, 1969, Ser. No. 850,861 Int. Cl. D21f 5/00 US. Cl. 162205 Claims ABSTRACT OF THE DISCLOSURE To improve the formation properties of a nonwoven web of cellulosic fiber, the web is dried to a moisture content of between l840% moisture and advanced through a pressure nip between a pair of opposed rolls. The rolls are each very slightly resilient but have a hard surface of between 0 and 10 P & I Plastometer A" ball).

The pressure nip compaction also contributes to densifi cation, improved surface smoothness and improved uniformity of fiber distribution.

BACKGROUND OF THE INVENTION In the manufacture of paper from cellulosic fibers, it is common to deposit from a headbox a fiber-water slurry (containing more than 99% water and less than 1% fibers) to form a web on a continuously moving fourdrinier wire. The wire retains most of the fibers while some of the water drains through the wire, and so when the web leaves the wire to enter a press section of the paper machine the water content of the web is generally 80-85% water. The web then passes through a plurality of dryer sections to complete the drying of the web before the web is wound for transport to an additional finishing or converting process. In papermakers jargon it is common to refer to the section of the machine between the headbox and press section as the wet end.

In the manufacture of paper it is highly desirable that there be a uniformity of fiber distribution throughout the web. In the language of papermakers it is common to use the word formation to describe the fiber distribution properties of the web. Thus, a paper having good formation is when the nonuniformity is the least. A paper having poor or wild formation is when there is nonuniformity in the fiber distribution throughout the web.

While there are machines, which will be referred to hereinafter, for evaluating formation properties, the judging of such properties by experienced papermakers is often accomplished by viewing a light through the translucent sheet of paper. If the sheet looks uniform, the formation is called good. If, however, the concentration of fibers appears denser in certain areas than other areas over an irregular pattern so that an irregular or blotchy structure is observed, then the formation is called poor or wild. Such blotchy structure, or irregular areas of fiber concentration, is sometimes referred to as fiocs because the nonuniform paper is said to be fiocculated.

Because of the desirability of making a paper which has good formation properties, there have been ways "developed in the past to improve formation. It is known that the consistency of the stock slurry in the headbox and on the forming wire (fourdrinier) affects formation. If the amount of water is increased the formation is usually improved, but there is a limit to how much water can be added because the machine speed would have to be slowed down to remove the excess water. If the web is too wet at the couch roll end of the wire, the web may break as it passes to the press section. As a further consideration, an increase in the amount of water on the wire increases the drying requirements through the dryer sections which may affect the speed that the web can be advanced.

Another mode which has been suggested to improve formation, especially in heavier sheets, is through the use of a dandy roll. The dandy roll is positioned over the fourdrinier wire and turns in light contact with the top of the web in what has been described as a light stirring action on the fibers. However, the use of a dandy roll has generally been limited to web speeds of less than 1,000 feet per minute.

It has also been suggested to improve formation by increasing the cutting action of refiners acting on the pulp before the pulp is made into paper. This cutting action lowers the drainage rate so that the fibers hold the water longer on the wire. Such lowered draining rate also requires slower running speed, and excessive cutting of fibers lowers strength properties thereof.

Other ways suggested in the past to improve formation, all of which are accomplished on the wet end of the machine, reside in controlling the velocity of the flow of the water-fiber mixture from the headbox to approximate the velocity of the forming wire and in providing a wire-shake mechanism for the forming wire itself.

Notwithstanding all of the above suggested ways to improve formation, it is desirable to have a way to improve formation by action on the web beyond the wet end of the paper machine so that if other methods are used on the wet end still further gains in formation improvement can be obtained; or, if desired, certain of the formation improvement operations on the wet end of the machine can be eliminated.

SUMMARY It is, therefore, an object of the present invention to provide a method and apparatus for improving the formation (uniformity of fiber distribution) of a wetlaid, nonwoven web by action that takes place on the web beyond the wet end of a paperforming machine, and thereby gain improvement in paper properties which accompany good formation.

Another object is to provide method and apparatus for treating a moving web to densify and smooth the web while improving the formation properties.

Still another object is to provide a method and apparatus for treating a nonwoven web so as to produce a web having potential to develop improved grease-resistant properties with a decrease in the amount of grease-resistant coating that need be applied to the web.

The above and other objects are obtained, according to a broader aspect of the present invention, by providing a method and apparatus for advancing the web at a moisture content of between about 18% and 40% through a pressure nip between a pair of pressure rolls. The surface hardness of the rolls at room temperature is such that the rolls are very hard but have some slight compressibility and resilience at the pressures used; and, more specifically, the roll surface hardness of each roll is between 0 and 10 P & I Plastometer ball).

BRIEF DESCRIPTION OF THE DRAWINGS An embodiment of the present invention is fully described below and is illustrated by the accompanying drawings in which:

FIG. 1 is a side view, mostly diagrammatic, of a paper machine with the apparatus of the present invention located therein;

FIG. 2 is a partially diagrammatic side view of the apparatus of the present invention in place between dryer sections of a paper machine;

FIG. 3 is a diagrammatic, sectional side view illustrating a web passing between pressure rolls, as provided by the present invention; and,

FIG. 4 is a graphical approximation of curves taken from charts of a QNS/M Formation Tester comparing the formation curves of a web given a treatment according to the present invention with a web not treated.

DETAILED DESCRIPTION First turning to FIG. 1 cellulosic pulp stock flows through a slice opening 11 in a headbox onto a traveling endless screen 12 called a fourdrinier wire which is entrained for rotation at one end about a breast roll 13 and at the opposite end about a couch roll 14. The stock in the headbox conventionally includes more than 99% water and less than 1% fiber.

Water is removed from the Web 15 through the screen 12, and the fibers become intermixed and matted together on the screen so that by the time the web reaches the couch roll 14 it has a water content of typically between about 80-85% water.

A felt press section 16 is conventionally used to remove additional water, and it is common for the web to contain from 60-70% water when it leaves the press section.

From the press section 16, the moving web is advanced through at least one dryer section 17 for removing additional moisture from the Web. This dryer section 17 or sections is used to bring the moisture content of the web down to between about 18-40% moisture.

The entire process which has just been described in connection with FIG. 1 is conventional, and it is to be understood that other apparatus or additional apparatus could be used for treatment of the web at any or all of the stations just described.

The present invention is concerned with apparatus and method for treatment of the web at a station indicated by the numeral 18 (FIGS. 1, 2 and 3). After treatment at station 18, the web may pass through an additional conventional dryer 19 or dryers and the web may then be wound into a roll 20. Of course, additional treatments such as calendering or coating could be applied to the web after the web leaves dryer 19.

At station 18 the web 15 is advanced over an upper guide roll 21 and then through a pressure nip 22 between a pair of opposed rolls 23 and 24; thence around a lower guide roll 25 to dryer section 19. It is the treatment supplied to the web in the pressure nip 22 that constitutes the inventive feature of the present invention; and, for such treatment to occur it is important that the web have a moisture content at time of treatment within specified ranges, and that the surface hardness of the rolls 23 and 24 each be within certain specified ranges. It is also important. that pressure be applied to the web 15 by the rolls 23 and 24 as the web is advanced through the nip, and the necessary pressure will be specified hereinafter.

The roll 23 is provided with an appropriate drive mechanism (not shown) to turn the surface of the roll in the direction of the arrow at the speed of the web 15 being advanced through the nip 22. The shaft forming the axis of the roll 23 is, of course, journalled for rotation and supportedat opposite ends thereof by an appropriate supporting means.

Opposite ends of the shaft forming the axis of roll 24 are journalled for rotation within bearing blocks 26, and these bearing blocks at opposite ends of the roll 24 are each integrally attached to a pivotable arm 27. The upper end of each arm 27 is pivotallysecured to a main horizontal supporting frame member 28 which is, in turn, supportedfrom the floor by vertical frame member 29. The lower end of each arm has a pivotal connection 30 to the end of a piston rod 31, the rod 31 being attached to a piston which is movable lengthwise within a cylinder 32 in accordance with fluid under pressure being supplied to 4 the cylinder at either of opposite ends of the piston by fluid lines 33 and 34. The end of the cylinder 32 remote from the piston rod is pivotally connected to a bracket 35 which is secured to vertical frame member 29.

While not specifically illustrated, it is desirable to provide a positive drive also for roll 24 to drive the surface of the roll at the speed of the moving web. This positive drive to both rolls 23 and 24 is especially desirable "at high papermaking speeds of, say, 1,500 ft./ min.

The roll 24 is, therefore, movable toward the roll so as to permit the surface of the roll 24 to be pressureloaded against the surface of roll 23 so that when web 15 is in the nip between the rolls there will be pressure exerted by the roll surfaces on the web 15.

The specific manner of supporting the rolls 23 and 24 is not critical, however, and any conventional supporting mechanism may be used so long as there is provided means to urge the surface of one roll in a direction toward the surface of the other roll to provide a pressure nip.

The surface hardness characteristics of each of the rolls is an important feature contributing to obtaining the treatment of this invention. In this latter regard, it has been found that the rolls should each have a surface hardness of between 0 and 10 P -& J Plastometer (%a" ball) at room temperature of 70 F. This does not necessarily mean that the rolls will be operated at room temperatures, but only specifies the surface hardness of the rolls when the rolls are at room temperature. With the hardness just specified, the rolls are very slightly compressible and slightly resilient at the nip pressures utilized. The preferred surface hardness range for each roll 23 and 24 is between 0 and 5 P & J Plastometer 0A3 ball) at room temperature.

In contrast to the present invention, it was tried to utilize one roll of the surface hardness indicated (between 0 and 5 P & J) and the other roll being between 10 and 15 P & J Plastometer, but the inventive results were not obtained. It was also tried to use one roll ofthe surface hardness indicated (between 0 and 5 P & J) and the other roll was brass (0 P & J), but the inventive results were not obtained with this combination either.

It is therefore important, as stated, that each roll 23 and 24 have a surface hardness between 0 and 10 P' & I Plastometer ball) and preferably between 0 and 5 P & J.

The roll may be constructed by applying, for example, a /8 inch thick covering material 36 (FIG. 3) having the desired surface hardness characteristics to the exterior surface of a steel core 37. It is desirable that the steel core be hollow so that a cooling fluidmay be circulated therethrough to provide some degree of cooling to the roll. This cooling effect is particularly desirable at high operating speeds where the nip action might cause undue heating and shorten the effective life of the rolls.

One material which has been found to be particularly desirablefor a roll cover 36 to provide the exterior surface for each of the rolls 23 and 24 is manufactured and sold by Stowe-Woodward Co. under the trademark Microrok. This material is a natural rubber compounded with a finely ground stone aggregate to provide the desired hardness properties.

The exact diameter of the rolls 23 and 24 does not appear to .be particularly critical and rolls of 12-inch and 23-inch diameter have been effectively used.

It has been stated above that the surface hardness of the rolls is important. This may be further expressed in terms of nip width of particular diameter rolls at particular loading pressures. Thus, the nip width should be between about .07 inch and about .42 inch when 23-inch diameter rolls are loaded to provide a nip pressure of about pounds per lineal inch. The nip width may be measured by using impression foil that is available for this purpose and pressure-loading the two rolls together (not rotating) with the foil in between. The impression width on the foil can be measured to obtain an indication of the nip width.

If the moisture content of the web at the time it enters the nip 22 is too low, the position of the fibres has already been set, and the improvement in formation does not occur. If the moisture content of the web 15 at the time it enters the nip 22 is too high, it has been found that irregular glossy spots, perhaps due to excess mobility of fibers and fibrils, are formed on the web so that the formation does not improve. The moisture content of the web should therefore be great enough so that a rearrangement of fibers and fibrils to a certain extent is possible, but not so great as to cause excessive, uneven glossy spots or mass movement of large massive areas. To accomplish the desired objective, then, the moisture content of the web should be between about 18-40%, and preferably between -35% when the web enters the nip 22 between the two rolls 23 and 24.

The pressure should be great enough to exert the desired compacting and fiber re-arranging action. It has been found that formation improvement is obtained with nip pressures as low as 50 pounds per lineal inch at the nip 22 between the rolls 23 and 24. The maximum pressure which may be utilized is not critical and depends mostly on the physical limitations of the structure of the rolls themselves. Pressures up to 1,350 pounds per lineal inch have successfully been tried. The desired amount of pressure utilized will vary with the basis weight of the web being treated, the heavier Webs utilizing greater pressure. The optimum pressure range for paper having a basis weight of 25-45 pounds/ 3000 ft. is between about 100 and 250 pounds per lineal inch at the nip 22.

While the exact action taking place on the web 15 in the nip 22 has not been definitely ascertained, reference to FIG. 3 may indicate what happens. First, assume the web 15 entering the nip 22 at 2535% web moisture content has a relatively rough surface and uneven formation. At this moisture content, the fiber structure of the web is a plastic mass which can be altered. Presenting this structure to the hard, but very slightly compressible, surfaces forming each side of the nip subjects the web to a combination of shearing forces (PS and PS and compressive stress (C). The compressive stress probably collapses the fibers 38, at least to a certain degree. The combination of forces results in what amounts to an extrusion of the web out of the nip to give a uniformly more compressed web with a smoother surface. Because of the slight resilience of each roll, there is a nip action from each roll causing the forces PS and PS to act on the web against the direction of movement, and this action is thought to contribute to fiber rearrangement in improving formation. If the moisture content of the web is too high (i.e., above about 40%) the web structure is too fluid and so the whole structure is displaced by mass movement of entire structural areas without differential fiber movement and so the formation is not improved.

As indicated previously, comparison of the uniformity of fiber distribution (formation) in sample sheets, can be judged by looking through the samples at a light source.

There are, however, laboratory instruments for automatically measuring and graphically recording formation in the form of a general formation curve. One such instrument is manufactured and sold by Electronic Automation Systems, Inc., of Grand Island, N.Y., and the instrument is known as the QNS/M Formation Tester. This instrument is described in an article entitled A Formation Tester Which Graphically Records Paper Structure, which appeared in the June 1960 issue of the publication Pulp and Paper Magazine of Canada.

-As indicated in the published article, the instrument graphically charts a formation curve and operates on the general principle of a scanning light source-photometer arrangement with the sample to be tested positioned between the light source and the photometer. The abscissa (X axis) of the plotted curve represents thirty steps,

which includes a calibration step as steps 1 and 30. Steps 3 through 28 utilize a filter in the machine at each step to provide a series of measurements corresponding to floc separation in inches (distance between centers of adjacent fiocs in the direction being scanned). For example, the filter at step 3 provides a reading which corresponds to a floc separation of 4.0 inches, the filter at step 16 provides a reading which corresponds to a floc separation of 0.21 inch (frequency of 800 cycles per second) and the filter at step 28 provides a reading which corresponds to 0.013 inch (frequency of 12,500 c.p.s.). Steps 2 and 29 provide what is described as an LIN reading. In the LIN position, the signal bypasses the filters so the reading represents the integrated variation represented by the individual filters.

The ordinate (Y axis) of the curve provided by the instrument just mentioned is the relative intensity 'of the signal seen by the photometer.

The curve which is plotted by the instrument, in testing a sample for formation, usually has its minimum ordinate, or Y axis values, at steps 3 and 28 (because the frequency of occurrence of flocs having the separations measured at steps 3 and 28 is not as great as at other steps) and the maximum ordinate value occurs at a filter-setting somewhere between steps 3 and 28.

As a general rule, it can be stated that if the curve recorded by the instrument is relatively flat, the formation (uniformity of fiber distribution) is good. The curve becoming more rounded or reaching greater extremes in the Y direction is indicative of poorer or wilder formation when comparing a given furnish at a given basis weight. The formation curve obtained with the QNS/M instrument can be further analyzed as follows:

(1) Overall formation (LIN value) .the integrated measure of the overall formation quality to which all irregularities contribute. The higher the value, the poorer or wilder the formation. This value is read directly from the ordinate of the curve.

(2) Predominant floc size index.the highest peak in the formation curve. The step in the X axis at which the highest peak occurs is the floc separation step that occurs most often (i.e., the floc size that is most predominant). As a general rule, the higher the value of the figure, as read on the Y axis, the poorer the forma tion. This value is also read directly from the curve.

(3) Skewness.the location of the predominant floc size relative to the step which is the center of the general formation curve (such center step being chosen as step 16 along the X axis of the curve). The skewness is equal to the step number where the predominant floc size occurs (highest peak) minus 16. For example, a positive skewness number indicates the predominant floc size is to the left of the center of the general formation curve and is in the range of floc sizes less than 0.21 inch. Having the predominant floc size occur in the range of small floc sizes is an indication of good formation. Therefore, as a general rule, the higher the skewness number, the better the formation.

The above discussion of curves obtained with a QNS/ M Formation Tester can be further illustrated by observing the curves charted at FIG. 4. In the solid line curve the LIN value is about 48 (relative intensity), the predominant floc size has a relative intensity value of about 46 and occurs at step 24 (X axis) so the skewness has a value of 8 (24-16). In the interrupted-line curve the LIN value is about 69, the predominant :floc size has a relativ intensity of about 59 and occurs at step 12 so the skewness has a value of 4 (12-16).

The following examples further illustrate the present invention. When high consistency refining is referred to, it means refining the pulp at about 30% consistency in a Bauer 411 double disc refiner in a manner more fully explained in United States Pat. No. 3,382,140. A conventional paperrnaking machine was used to form the pulp into paper at machine speeds of about 1,500 ft./min. except that an apparatus according to the present invention was positioned at station 18 (FIGS. 1-3) for treatment of the web according to the present invention. In the tables that follow, when a .nip pressure of is indicated, it means that the paper web was not given an inventive treatment. When a positive nip pressure is indicated in the tables, it means that the moving paper web was passed between the rolls 23 and 24 (FIGS. 1, 2 and 3) and the rolls were urged together at the indicated pressure. When the web moisture is indicated in the tables, it identifies the moisture content of the paper web when the web enters the nip between rolls 23 and 24.

In the examples and tables which follow, various tests were performed using standard TAPPI methods, unless otherwise indicated.

Example 1 A pulp containing 60% Douglas fir kraft, bleached to a brightness of 80 G.E.R.S. (General Electric Recording Spectrophotometer), and 40% bleached cottonwood sulfite was refined at high consistency (power input to refiner of 10.8 horsepower day/air dry ton of pulp) to 442 cc. CSF (Canadian Standard Freeness). The consistency was then reduced for conventional Claflin refining to 330 cc. CSF, followed by conventional Jordan refining to 195 cc. CSF.

The pulp was diluted to conventional papermaking consistency and formed into paper on a papermaking machine such as diagrammatically illustrated at FIG. 1. The basis weight of the paper formed was about 32 lb./ream (3000 wt. The moisture content of the web entering the nip between rolls 23 and 24 was about 28.0%. During a part of the run the web bypassed the nip between rolls 23 and 24 (O nip pressure so no inventive treatment) and during other parts of the run the web was fed through the nip between rolls 23 and 24 at a pressure of about 175 lbs/lineal inch. After paper had been made, samples were taken from the formed paper corresponding to locations where the nip pressure was zero (no inventive treatment) and also at locations where the nip pressure of 175 p.1.i. was used (inventive treatment). The samples were tested in the QNS/M Formation Tester, as referred to above, and an approximation of the graph appears at FIG. 4 of the drawing. As discussed above, the solid line curve of FIG. 4 indicates substantially improved formation as compared to the interrupted-line curve (no inventive treatment). In addition, the samples (inventive treatment vs. no inventive treatment) were visually judged by holding the samples to a light, and the formation appeared significantly improved on the paper that was given an inventive treatment.

Example 2 A pulp containing 60% strong bleached sulfite, bleached to 81-84 G.E.R.S. (predominantly hemlock), and 40% ENSO birch kraft, bleached to 8-5 G.E.R.S., was refined at high consistency (power input to refiner of 7.1 horsepower day/air dry ton pulp) to 460 cc. CSF (Canadian Standard Freeness) followed by conventional Claflin and Jordan refining to 357 cc. CSF and 235 cc. CSF respectively. The pulp was dilutedand made into paper as in Example 1 except that the moisture content at the nip between rolls 23 and 24 was slightly different, and a broader range of nip pressures was utilized. Samples of the paper were tested on the QNS/M Formation Tester and other of the samples were tested for paper properties. The curves obtained by the QNS/M Formation Tester were analyzed visually and also by computer technique (which tended to smooth out the curves somewhat, but still approximated the actual curves taken from the tester). For the formation analysis appearing in Table I below, the computerized values of the curves were used. The paper properties measured in Table I were, as indicated, at various nip pressure loadings taken from the paper produced according to Example 2.

TABLE I Sample A B C D E Nip pressure, lb./1ineal inch 0 175.5 405 810 1,350 Web moisture, percent 30.0 30.0 37.0 35.0 35.0 Formation analysis:

Overall index (LIN value) 79 55 53 54 57 Predominant fioe size index- 58. 34 39. 92 37. 14 37 68 38. 61

skewness 4 6 7 1 8 Paper properties measured: 1

Basis weight, lbs/ream (3,000 ft?) 33. 4 33. 5 33.0 33. 2 33. 3 Caliper. 0.001" 3. 00 2. 40 2. 36 2. 32 2. 34 Density, grams/cc 713 893 894 916 910 Tensile, lbs/inch (Instron):

WMD 28. 6 30.09 29; 1 28. 6 29. 5

OMD 7.8 7.4 7.4 7.7 TEA (Tensile Energy (Instron):

WMD 6.7 6. 8 6. 2 5. 9 6. 3 D 7.8 9.9 9.4 9.0 9.7 Tear, grams/she MD- 38.0 32.2 31.4 31.1 30.9 CMD 47. 1 40. 4 38. 8 38. 8 38. 0 smoothness, ec./miu.

(Sheffield):

ToP 280 168 173 166 165 Wire 319 173 175 171 172 Opac y, percent (Banseh & Lomb 49. 4 45.0 43. 5 44.3 43. 9 Densometer, cc./min.

(Sheffield) 31 14 10 11 10 Oil absorption, pereentuh, 14.5 8. 4 8.1 7. 8 8. 2 Internal bond, ft.-lbs./in.

(Scott Tester) .386 462 .427 .460 489 Example 3 A pulp containing 60% Douglas fir kraft, bleached to a brightness of G.E.R.S., and 40% ENSO birch kraft, bleached to G.E.R.S., was refined at high consistency (power input to refiner of 9.2 horsepower day/air dry ton pulp) to 480 cc. CSF, followed by conventional Claflin to 345 cc. CSF and Jordan to 231 cc. CSF. The pulp was made into paper and samples obtained substantially in the same manner as was reported in'regard to Example 2 above. The data obtained from the samples is indicated at the following table.

TABLE II Sample F G H Nip Pressure, lb./1ineal1nch 0 175.5 405 Web moisture, percent 36. 0 36. 5 33. 0 Formation analysis:

Overall index (LIN value) 84. 0 55. 0 59. 0

Predominant fioe size index 62. 05 39.06 40. 21

skewness 5 8 12 Paper properties measured:

, Basis weight, lbs/ream- 33.1 32. 5 33.0 Caliper, 0.001 3. 28 2. 46 2. 50 Density, grams/cc" 646 845 845 Tensile, lbs/inch WMD 32. 9 83. 9 13. 6 14. 2

172 177 159 159 51. 6 49. 3 31 25 Oil absorption, percent. 19. 59 11. 84 23 Internal bond, it. lbs./in. 308 363 423 Example 4 Substantially the same procedure as Example 2 was followed except in this Example 4 the pulp used was a mixture of 15% Douglas fir kraft, bleached to 80 G.E.R.S. brightness, 45% strong bleached kraft, and 40% ENSO birch kraft. The pulp was refined at high consistency (power input of 8.3.5 horsepower day/ air dry ton pulp) to 420 cc. CSF, followed by Claflin to 351 cc. CSF and Jordan to 222 cc. CSF.

The pulp was made into paper, and samples were obtained substantially in the same manner as was reported in regard to Example 2 above. The data obtained from the samples is indicated atthe following Table III:

TABLE III Sample I J i K Nip pressure lb./lineal inch. 175. 405 Web moisture, percent 28 33 36 Formation analysis: 1

Overall index (LIN value) 67 52 53 Predominant fioc size index .t 51. 83 46. 08 40. 47 Skewness 5 12 Paper properties measured:

Basis weight, lbs/ream 34. 0 34. 0 34. 9 Caliper, 0.001 3. 20 2. 38 2. 40 Density, grams/cc 680 914 931 Tensile, lbs./inch:

WMD 30. 5 30. 6 31. 9 3 12. 3 13. 6

.7 37. 2 37. 8 .6 48. 9 48. 4 smoothness, cc./min.:

To 290 152 151 Wire 323 134 133 Opacity, percent 58. 3 53. 5 53. 7 Densometer, ce./min 61 41 33 Oil abosrption, percen 18. 60 11.35 10. 37 Internal bond, tt.-lbs./in. 329 387 386 Example 5 This example is presented to compare the grease-resistant properties of a web which has been given a rollcompacting treatment according to the present invention with a web which has not been given the roll-compacting inventive treatment, after both such webs having also been given a conventional size press barrier coating. This illustrates the inventive process requires less coating material to obtain even greater grease-resistant properties than a web which has not been given the inventive treatment.

Samples from Table I, Sample A (not having inventive treatment), and Table I, Sample B (having inventive treatment), were each given a conventional size press coating of a solution of 10% Penford Gum 280 in 90% water. Penford Gum 280 is a hydroxy ethyl ether derivative of corn starch and is sold by Penick and Ford Limited of Cedar Rapids, Iowa. When each of these samples was given the same coating treatment, it was found that Sample A picked up 2.7 lb./ream coating while Sample B picked up 1.6 lb./ream coating.

After coating each Sample A and Sample B, the samples were given a grease-resistance test with a 3M (Minnesota Mining & Manufacturing Co.) Kit used for this purpose. The 3M Kit has twelve Kit numbers (Kit Nos. 1-12), each containing varying amounts of castor oil, toluene and heptane. In Kit No. 1, the volume of castor oil is 200. with zero toluene and heptane. The volume of castor oil decreases and the volume of toluene and heptane increases gradually through the sequential Kit Numbers, and Kit. No. 12 has a volume of zero castor oil, 90 toluene and 11 0 heptane. The 3M Kit value is defined as the highest Kit Number solution that will stand on the surface of a sheet for 15 seconds in the form of a drop without failing. The idea is that the higher the Kit Number a paper can stand without failure, the better the oil resistance.

As stated, both Samples A and B were given a size press coating. Then, each sample was given a conventional machine finish (calendering operation) after coating. The calendered Sample A had a 3M Kit Rating of 4, but the calendered Sample B had a 3M Kit Rating of 8.

Additional coated Samples A and B were, instead of being calendered after coating, dampened and supercalendered, and a coated, supercalendered Sample A had a 3M Kit Rating of 3, while a coated, supercalendered Sample B had a 3M Kit Rating of 5.

Wicking resistance of Samples A and B (obtained from Example 2 above) was measured using a colored turpentine solution. The paper was cut into 1 x 6 inch strips and Kit Rating and a low wicking value.

The results just reported in connection with Example 5 indicate that paper treated according to the present invention has "improved capability of developing good grease resistance when given a size press coating.

While the inventive treatment in itself does not produce a slgnificant improvement in greaseproofing effect until a barrier coating is applied, there is a positive correlation between oil absorption of a base paper and grease resist ance of the paper after treatment with a size press formulation. Oil absorption properties then become a convenient tool to use in judging the base papers value for potential greaseproof applications without going through the actual step of applying a barrier coating. It is noted that in all examples the inventive treatment improved the oil absorption properties.

When the QNS/M Formation Tester was used to graphically judge the samples in the examples above, the data was obtained from a CMD (cross machine direction) scan of the sample. Improvements were also noted in scanning the sample in a machine direction (WMD). Also, the improved formation was apparent in the invention-treated samples by visual inspection thereof.

Cellulosic material which is useful as a starting material in accordance with this invention may be derived from any species of coniferous pulp wood, such as spruce, hemlock, fir, pine and others; deciduous pulp wood, such as poplar, birch, cottonwood, alder and others; as well as from fibrous, nonwoody plants suitable for paper making, such as cereal straws, bagasse, corn stalks, grasses and the like, and also the usual waste cellulosic sources. Blegds of pulp from the mentioned sources may also be use It is to be noted that an improvement in formation was noted in connection with each of the pulp sources utilized in Examples 1-4 when treated according to the inventive process.

In addition to the improved formation (uniformity of fiber distribution), the web was, in each instance, made smoother and densified by the inventive process.

Also, it is to be noted that while there is some slight loss in tearing strength in a paper treated according to this invention, this loss is outweighed by a general increase in other strength factors such as tensile and internal bond, and, as a general rule, tensile energy absorption and stretch. Further, the opacity is reduced to gain in transparency, which is desirable in certain papers. Of course, the invention would also have application to papers which were purposely loaded with opacifying agents.

While good formation (uniformity of fiber distribution) is an important consideration in any papermaking operation, it is especially important in expensive quality papers. Carbon paper requires good uniformity of fiber distribution because nonuniformities result in a mottled appearance after carbon coating, High quality bond and business form papers require a uniform sheet to obtain maximum strength with light basis weights. Further, such uses as tracing papers require uniform transparency, and the paper must have uniform formation.

While the foregoing specification has set forth embodiments of the present invention in considerable detail for purpose of making a complete disclosure thereof, various other embodiments and modifications will occur to those skilled in the art, but will fall within the spirit and scope of the invention defined in the following claims.

What is claimed is:

1. A method of improving the uniformity of fiber distribution in a process for forming a wet-laid web of nonwoven fibers wherein the improvement comprises:

(a) advancing'the'web at a'moisturecontent between- 18 and 40% 'moisture'through a nip provided by a pair of rolls wihch are slightly compressed and resilient'=-at a minimum pressure of 50 pounds per lineal inch exerted on the-=rolls' by urging them toward each other,' .each of the rolls having a room temperature -(70fr-F.) surface hardness of between and 10 P &'J Plastometer /s' ball) but not as hard as zero P & J; and" I (b) applying pressure greater than 50 pounds per lineal inch to the web through the medium ofthe rolls with the surfaces of the rollstdirectly contacting opposite surfaces of the web as the web is advanced through the nip between the rolls.

2. The method as set forth in claim 1 wherein the moisture content of the web is between 25-35% moisture when the web enters the nip between the two rolls.

3. The method as set forth in claim 1 wehrein the pressure is between 100-250 pounds per lineal inch, and the web has a basis weight between 25-45 lbs./ 3000 ft.

4. The method as set forth in claim 1 wherein the surface hardness of the rolls is between 0 and 5 P & J.

5. The method as set forth in claim 1 wherein the moisture content of the web when the web enters the nip between the rolls is 25-35% moisture, and the surface hardness of the rolls is between 0-5 P & J when the rolls are at room temperature (70 F.).

6. The method as set forth in claim 1 wherein surface hardness of the rolls is such as to provide a nip width of between .07 inch-.42 inch when the two rolls are 23 inches in diameter and urged together under a pressure loading of about 165 pounds per lineal inch.

7. A method of making a web of nonwoven cellulosic fibers by wet-laying a fiber-liquid slurry on a moving forming wire and thereafter removing moisture from the slurry to form a finished web, wherein the improvement comprises:

(a) densifying, smoothing and improving the uniformity of fiber distribution in the web by (i) drying the web to a moisture content of between 18 and 40%;

(ii) advancing the web at said moisture content of 18-40% through the nip between a pair of rubber-like rolls which are slightly compressed and resilient at a minimum pressure of 50 pounds per lineal inch exerted on the rolls by urging them toward each other, each of the rolls having a room temperature (70 F.) surface hardness of between 0 and P & J Plastometer /s" ball) but not as hard as zero P & J; and

12 (iii) applying pressure of at least 50 pounds per lineal inch to the web. through the medium of the presure rolls with the surfaces of the rolls directly contacting opposite surfaces of the web.

8. Thesmethod as set forth in claim'7 wherein a greaseresistant barrier coating is applied to a surface of the finished web. I 9. In an apparatus for continually forming a continuously'moving wet-laid web of nonwoven fibers and drying the web by advancing the web through a plurality of dryer sections wherein the improvement comprises:

(a) a pair of cylindrical rolls positioned to define a pressure nip between dryer sections where the web has a moisture content between 18 and 40%, said rolls each being of a rubber-like material and slightly compressible and resilient at pressures of at least 50 pounds per lineal inch exerted on the rolls by urging them toward each other, each roll having a room temperature F.) surface hardness between 0 and 10 P & J Plastometer ball) but not as hard as zero P & J;

(b) means for directing the web through a nip between the rolls; and

(0) means for urging the surface of said rolls together at pressures greater than 50 pounds per lineal inch, whereby the pressure nip between the rolls is adapted to receive the Web and redistribute the fibers thereof to improve the uniformity of fiber distribution in the web.

10. The apparatus as set forth in claim 9 wherein the surface hardness of each of said rolls is between 0-5 P & J.

References Cited UNITED STATES PATENTS 3,104,197 9/1963 Back et al 162-206 X FOREIGN PATENTS 522,196 6/ 1940 Great Britain. 696,733 11/1964' Canada,

OTHER REFERENCES Britt: Handbook of Pulp and Paper Technology, Reinhold Publishing Corp. (1964), pp. 380-381.

S. LEON BASHORE, Primary Examiner R. H. TUSHIN, Assistant Examiner US. Cl. X.R. 162-206, 361 

